Hereditary retinal diseases lead to a progressive loss of photoreceptors often causing blindness in the patients’ middle age without an available therapy. Microelectronic subretinal implants are designed to replace the function of the degenerated photoreceptors.

Methods

In the monocentric part of our clinical trial the subretinal Alpha IMS microelectronic visual implants (Retina Implant AG, Reutlingen, Germany) had been implanted in 11 patients in end stage retinitis pigmentosa. The implant's core is a subretinal chip with 1500 pixels. Each pixel has a photodiode, a differential amplification and an electrode for charge transfer to the adjacent retinal layers. Brightness gain and contrast can be adjusted by the patient manually via two knobs on the external handheld unit. By means of an electroretinographical setup (Espion, Diagnosys LLC, Cambridge, UK) light responses from chips were measured on the study eye in various gain-contrast settings in a special protocol of 9 steps while raising stimulus intensities from 0.1 photopic cd/m2 to 1000 photopic cd/m2 by 0.5 log steps.

Results

The typical voltage versus log light intensity response function of the subretinal implant for each contrast sensitivity and gain combination has a sigmoidal shape. Variation of the chip sensitivity shifts the curve left or right and allows to define the optimal adjustment of all 1500 amplifiers in the chip to a given luminance range of the ambient lightening. Variation of the output gain changes the maximal output of the chip in the plateau of the curve in order to adjust the point of saturation. Variations among implants are relatively minor.

Conclusions

The function of subretinal visual microelectronic implants can be measured objectively by special protocols using standard electroretinographical equipment. The exact knowledge about implant transfer characteristics is necessary for optimal parameter setting of artificial vision devices, especially in the training period after implantation as well as for objective measure of the chip function postoperatively. Suboptimal results in perception contrast and saturation can be avoided by assessing this curve in situ and using of appropriate gain-contrast settings.